For controlling the energy stored in an inductive load, it is important to make available a diagnostic signal when the current in the inductor reaches a preset level.
A driving system for an inductor is depicted in FIG. 1. The inductor L is connected between a supply node Vs and a power transistor T.sub.1 that acts as a switch, driven by a driver circuit (DRIVER) to an input of which a signal V.sub.IN is applied.
As shown in FIG. 2, when V.sub.IN goes high, T.sub.1 turns-on and a current I.sub.c starts to flow through the inductor, increasing with a linear law in function of time. For any value I reached by the current I.sub.c, the corresponding energy that is stored in the inductance is given by: ##EQU1##
The power stage normally comprises a circuit for limiting the maximum current in order to avoid destruction of the transistor. Therefore in the diagram of FIG. 2, the current is limited to a maximum value I.sub.max.
Moreover, if the transistor T.sub.1 must be turned-off always at the same level of energy stored in the inductor, it is necessary to produce a signal when the current I reaches a preset level I.sub.D. That level is chosen in order to optimize the energy stored in the inductor at the end of each charging phase, that is when T.sub.1 turns off. This is often required, for example, in electronic spark-plug driving systems.
In the latter systems it is also necessary that the level I.sub.D at which the turning off of the transisotor T.sub.1 occurs be very close to the maximum current I.sub.max.
In conventional systems, these requirements are achieved in the manner depicted in FIG. 3. A maximum current limiting circuit A.sub.1 (LIMITER) acts on the driver circuit (DRIVER) when the voltage drop across the sensing resistance R.sub.s, due to the current I.sub.c, equals the reference voltage E.sub.1, that is when R.sub.s *I.sub.max =E.sub.1. Similarly, a diagnostic signal, shown as V.sub.D in FIG. 2, is produced when R.sub.s *I.sub.D =E.sub.2. It is worth to be considered that, for the above reported reasons, I.sub.D may have a value very close to I.sub.max. For this purpose, as shown in FIG. 3, a second diagnostic comparator (DIAGNOSTIC) A.sub.2 is employed.
Since I.sub.D must be lower than I.sub.max, though close thereto, E.sub.1 must also be greater than E.sub.2 but must have a value very close thereto. The absolute values of E.sub.1 and E.sub.2 should on the other hand be as low as possible, because they correspond to a voltage drop on R.sub.s due to the current I.sub.c. Such a voltage drop is in series with the saturation voltage of the transistor T.sub.1 and causes power dissipation. For this reason the voltage on R.sub.s, and therefore also E.sub.1 and E.sub.2, should not be greater than few tens of mV. This means that if a diagnostic signal, for example greater than 90% of the limit current I.sub.max, is required, the relation E.sub.2 &gt;0.9*E.sub.1 must be verified. As a numeric example, by assuming I.sub.max =5 A and R.sub.s =10 m.OMEGA., it will be E.sub.1 =50 mV and therefore E.sub.2 &gt;0.9*50 mV=45 mV. This means that E.sub.1 -E.sub.2 &lt;5 mV. Occasionally, E.sub.2 &gt;0.95*E.sub.1 may be required and the difference between E.sub.1 and E.sub.2 must be even smaller than few mV.
The known arrangement of FIG. 3 is critical, because the voltage drop on R.sub.s at which the operational amplifiers A.sub.1 and A.sub.2 must react is very small (in the order of millivolts) and is comparable, in terms of order of magnitude, with the voltage offset of the comparators that are employed. This may determine a non-negligeable imprecision in signalling the reaching by the current I.sub.c of the diagnostic level I.sub.D. Eventually, if the offset of the differential amplifier A.sub.2 become greater in absolute value than the voltage difference E.sub.1 -E.sub.2, the system will not produce the required diagnostic signal, with serious consequence on the functioning of the system.
In other words, in all the applications where, for obvious reasons of optimization, the current level I.sub.D must be fixed very close to the limit level I.sub.max, the known circuits may be operating in extremely critical conditions. They may therefore lose in reliability and precision in ensuring a correct ratio between I.sub.D and I.sub.max.